1. Field of the Invention
The present invention relates to a method for evaluating a crystalline semiconductor substrate, and more particularly to a method for evaluating a crystalline semiconductor substrate which includes a collector layer, a base layer, and an emitter layer and is used for heterojunction bipolar transistors.
2. Background Art
Heterojunction bipolar transistors (hereinafter referred to as HBTs) are widely used for power amplifiers for portable telephones, etc. since they provide good high-frequency characteristics and high current density. As its emitter-base junction, the HBT employs a heterojunction in which the emitter layer has a band gap larger than that of the base layer to enhance the emitter injection efficiency of the bipolar transistor. A semiconductor device made up of HBTs employs a semiconductor crystal substrate having a multilayer structure.
With reference to FIG. 9, a description will be made of a general cross-sectional structure of the semiconductor crystal substrate for HBTs, using an AlGaAs HBT as an example. As shown in the figure, the AlGaAs HBT includes a semiconductive GaAs substrate 32, and an n-GaAs subcollector layer 33, an n-GaAs collector layer 34, a p-GaAs base layer 35, an n-AlGaAs emitter layer 36, and an n-GaAs contact layer 37, which are all formed on the semiconductive GaAs substrate 32 in that order. These layers are formed by epitaxially growing each layer by use of, for example, the metalorganic chemical vapor deposition method (hereinafter referred to as the MOCVD method). Further in FIG. 9, reference numeral 38 denotes collector electrodes; 39 denotes base electrodes; and 40 denotes an emitter electrode. The collector electrodes 38 have a laminated structure made up of, for example, AuGe/Ni/Au. The base electrodes 39, on the other hand, have a laminated structure made up of, for example, Pt/Ti/Au. Furthermore, the emitter electrode 40 is made of, for example, WSiN.
To enhance the high-frequency characteristics of an HBT configured as described above so that its characteristics are sufficient for a microwave device, the base resistance must be reduced by reducing the thickness of the p-type compound semiconductor crystal layer constituting the base layer and increasing the impurity concentration. For example, a known method for reducing the base resistance is to add, as an impurity, carbon to the p-GaAs layer, which is a p-type compound semiconductor crystal layer used as the base layer, in order to increase the carrier concentration of the base layer. In this method, however, hydrogen is undesirably taken into the base layer from ambient atmosphere in the base layer growth process. If hydrogen is included into the base layer, an initial change in the electrical characteristics, especially in the current gain is observed, which is disadvantageous to the quality control. This phenomenon is explained below using a specific example.
FIG. 10 shows the change in the current gain (xcex2) of an HBT with changing base current (Ib). The HBT indicated by the figure has a base layer whose carrier concentration and hydrogen concentration are approximately 4xc3x971019 cmxe2x88x923 and 2xc3x971019 cmxe2x88x923, respectively. The thickness of the base layer of the HBT is approximately 1,000 xc3x85, and its emitter size is 4xc3x9720 xcexcm. The change in the current gain (xcex2) was measured five times on the same conditions. In the figure, the label xe2x80x9cFirst measurementxe2x80x9d indicates the characteristic curve measured for the first time immediately after the device was produced, while the label xe2x80x9cFifth measurementxe2x80x9d indicates the characteristic curve measured for the fifth time.
As shown in FIG. 10, the current gain (xcex2) changes with changing base current (Ib). Specifically, when the base current (Ib) is increased, the current gain (xcex2) increases to a certain value and then decreases. The shapes of the curves of the current gains (xcex2) measured immediately after energization for the first time and the fifth time are greatly different from each other when the current gains (xcex2) increase. Specifically, the current gain (xcex2) increases more rapidly as the number of times the device is energized increases. However, the maximum value of the current gain (xcex2) measured for the first time is not largely different from that measured for the fifth time. Furthermore, the shapes of the curves obtained when the current gains (xcex2) decrease are substantially the same.
The occurrence of the phenomenon shown in FIG. 10 that the current gain increases more rapidly with increasing number of energization operations is conceivably attributed to hydrogen included in the base layer of the HBT. That is, inclusion of hydrogen into the base layer of the HBT makes the electrical characteristics of the device extremely unstable, which is disadvantageous to the quality control of the semiconductor device. On the other hand, the change in the current gain with increasing number of energization operations becomes small for the fifth and later measurements, making the characteristics stabilized. However, inspecting the product after its characteristics have become stable takes considerable time, which is not preferable in terms of productivity.
Furthermore, conventionally, it is difficult to measure an initial change in the current gain at the time point when the crystal has been grown. That is, it is not possible to determine the initial change in the current gain until an HBT device is actually manufactured (from the grown crystal) and its electrical characteristics are evaluated. Such characteristics (as the current gain change) of a semiconductor crystal substrate cannot be determined without actually manufacturing an HBT device from it, raising the problem that it is not possible to perform the quality control at stages before the HBT is manufactured from the semiconductor crystal substrate.
The present invention has been devised in view of the above problems. It is, therefore, an object of the present invention to provide a method for evaluating a semiconductor crystal substrate in such a way that it is possible to estimate the initial change in the current gain of the semiconductor crystal substrate.
Another object of the present invention is to provide a method for evaluating a semiconductor crystal substrate in such a way that it is possible to perform quality control of an HBT device.
Other objects and advantages of the present invention will become apparent from the following description.
According to one aspect of the present invention, in a method for evaluating a semiconductor crystal substrate which includes a collector layer, a base layer, and an emitter layer and is used for a heterojunction bipolar transistor, a semiconductor crystal substrate to be evaluated which includes a crystal layer whose composition is the same as that of the base layer is produced. Excitation light is irradiated to the semiconductor crystal substrate to be evaluated and a change with time in an intensity of photoluminescence from the crystal layer is measured before the intensity becomes saturated. A change with time in a current gain of the heterojunction bipolar transistor produced using the semiconductor crystal substrate is measured based on the change with time in the intensity.
Other and further objects, features and advantages of the invention will appear more fully from the following description.